Update README.md

README.md

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-# Swiftmokey
-Monkey Language written in Swift
+# SwiftMokey
+An interpreter for the Monkey programming language written in Swift.
+
+What is monkey language ?
+---------
+Monkey is a programming language used in a book ["Writing An Interpreter In Go"](https://interpreterbook.com/) by Thorsten Ball.
+
+Language Features
+- C style syntax
+- variable bindings
+- integers and booleans
+- arithmetic expression
+- builtin function
+- first class and high-order function
+- closure
+- string , array , hash data structure
+
+Example
+---------
+variable
+```
+let price = 100;
+let name = "Ter";
+let discountPrice = price - 20;
+```
+arithmetic
+```
+2 * 3 + 1 // => 7
+-2 + 3 // => 1
+2 * (5 + 10) // => 30
+50 / 2 * 2 + 10 // => 60
+```
+
+boolean
+```
+!true // => false
+1 < 2 // => true
+1 == 2 // => false
+1 != 2 // => true
+true == true // => true
+```
+
+condition 
+```
+if ( 1 < 2 ) { 
+  return 10 
+} else {
+  return 20
+}
+```
+implicit return
+```
+if ( 1 < 2 ) { 
+  10 // don't need to write return
+}
+```
+
+function
+```
+let add = fn(x , y) { x + y }
+add(2, 3) // => 5
+```
+
+high-order function
+```
+let adder = fn(x) { fn(y) { x + y } };
+let addTwo = adder(2);
+addTwo(3) // => 5
+
+let add = fn(a, b) { a + b };
+let sub = fn(a, b) { a - b };
+let apply = fn(a, b, func) { func(a,b) };
+apply(2, 2, add); // => 4
+apply(10, 2, sub); // => 8
+```
+
+string and built-in function
+```
+let a = "Hello"
+let b = "World!!"
+let helloWorld = a + " " + b // => Hello World!!
+
+len("Hello") // => 5
+len(a) // => 5
+```
+
+array and built-in function
+```
+let numArr = [1 + 1, 2 * 2] // => [2, 4]
+
+let index = 1;
+let arr = ["one", "two", "three"]
+
+arr[0] // => one
+arr[index] // => two
+arr[1 + 1] // => three
+
+first(arr) // => one
+last(arr) // => three
+rest(arr) // => ["two", "three"]
+let newArr = push(arr, "four") // => ["one", "two", "three", "four"]
+```
+
+hash (int ,string ,boolean as key)
+
+```
+let two = "two";
+let h = { "one" : 1, 
+          two : 1 + 1, 
+          "thr" + "ee": 3,
+          4 : 4,
+          true: 5,
+          false: 6
+        };
+h["one"] // => 1
+h["two"] // => 2
+h["three"] // => 3
+h[4] // => 4
+h[true] // => 5
+h[false] // => 6
+
+len(h) // => 6
+```
+Demo Program
+
+```
+let map = fn(arr, f) {
+ let iterator = fn(arr, accumulated) {
+   if (len(arr) == 0) {
+     accumulated
+   } else {
+     iterator(rest(arr) , push(accumulated, f(first(arr))));
+   }
+ };
+ iterator(arr, []);
+};
+
+let a = [1, 2, 3, 4];
+let doubleValue = fn(x) { x * 2 };
+map(a, doubleValue); // => [2, 4, 6, 8]
+```